1. Field of the Invention
The present invention relates to a direct current to direct current converter used for instance in an auxiliary direct current source in an electric train.
2. Related Art Statement
It has been known a double step-down chopper device as shown in FIG. 4, when a chopper device is used for an auxiliary direct current source of for example, in an electric train.
In FIG. 4, reference numeral 1 shows a direct current source, 2 and 9 reactors, 3, 4 and 10 condensers, 5 and 6 switching elements, 7 and 8 diodes and 11 shows the load.
Such a circuit arrangement acts to divide the voltage of the direct current source 1 into two substantially equal voltages by the condensers 3 and 4 and by alternately switch on and off the two switching elements 5 and 6 generally at 180.degree. phase shift, a direct current to direct current converter is realized by means of the switching elements having relatively lower withstand voltages compared with the voltage of the direct current source 1 and by means of reactors having smaller capacities compared with the switching period of the switching elements. Although a very detailed explanation is omitted since the device has been conventionally used and well known in the art, but the function thereof will be explained by referring to FIG. 5.
FIG. 5 shows waveforms of various portions of the circuit shown in FIG. 4. Namely, G.sub.1 and G.sub.2 show gate signals of the switching elements 5 and 6, respectively. V.sub.1 and I.sub.1 are the voltage and current, respectively, across the reactor 9. V.sub.0 is the voltage of condenser 10 consisting the direct current output voltage. FIG. 5a shows a case in that the voltage V.sub.0 of the condenser 10 is lower than the voltage of condensers 3 and 4 and FIG. 5b shows a case in which the voltage V.sub.0 is higher than the voltage of the condensers 3 and 4 and is lower than the voltage V.sub.D of the direct current source 1.
At first referring to FIG. 5a, if the switching element 5 turns-on at a time T.sub.0, a closed circuit is formed in the following chain. ##STR1##
If we assume the voltage of the condenser 3 as; EQU V.sub.D1 [.div.(V.sub.D /2)],
a difference of voltages between this voltage V.sub.D1 and the voltage V.sub.0 across the condenser 10 is applied across the reactor 9. Accordingly, an increasing current having gradient of increase determined by the reactance L, i.e. ##EQU1## flows through the reactor 9 and the reactor 9 stores the power while supplying the power to the load 11.
In this case, the power is supplied from the direct current source 1 forming an input of the device through the following route. ##STR2## This condition continues until a time T.sub.1.
If the switching element 5 turns-off at the time T.sub.1, a closed loop circuit is formed in the following route. ##STR3## So that the reactor 9 produces a voltage having the same value with that of the condenser 10 and having the opposite polarity with that during the term (T.sub.0 -T.sub.1) and a decreasing current decided according to the following equation; ##EQU2## flows through the reactor 9 and it discharges the power stored in it to the load 11. In this case the power is supplied from the direct current source 1 likewisely the previous term of (T.sub.0 -T.sub.1).
During the succeeding term after this time T.sub.2, a same operation is repeated by the switching element 6 and the condenser 4.
Among the whole period of term, during the terms (T.sub.0 -T.sub.1) and (T.sub.2 -T.sub.3), the power is supplied from the direct current source 1 to the condenser 10, i.e. load 11, the voltage V.sub.0 tends to increase depending on the value of the condenser 10 and the load 11. Whereas, during the terms (T.sub.1 -T.sub.2) and (T.sub.3 -T.sub.0), since the reactor 9 and the condenser 10 supply the power to the load 11, the voltage V.sub.0 tends to decrease. Therefore, the voltage V.sub.0 is controlled at a mean value between the maximum increased voltage and the minimum decreased voltage throughout the overall period.
Then referring to a case shown in FIG. 5b, if the switching element 6 turns-on at a time T.sub.4, as the switching element 5 had already been turned-on in this mode, following closed circuit is formed. ##STR4##
If we assume the voltage across the condenser 4 as V.sub.D2 [.div.(V.sub.D /2)], an increasing current having its gradient decided by the sum of voltages V.sub.D1 and V.sub.D2 and deducted by the voltage V.sub.0 and divided by the value of the inductance L, i.e. ##EQU3## will flow through the reactor 9, so that a power is stored in the reactor 9 while supplying the power to the load.
When the switching element 6 turns-off at a time T.sub.5, following closed circuit is formed. ##STR5## Thus a decreasing current having gradient decided by the difference of voltage V.sub.0 and V.sub.D1 [.div.(V.sub.D /2)] and the value of the inductance L will flow through the reactor 9 so that the rector 9 discharges the power stored in it. After the time T.sub.6, a same operation will be repeated by the condenser 4 and the switching element 6.
The operation mode of supplying the power from the direct current source 1 and the controlling of the voltage V.sub.0 is the same as the case of FIG. 5a.
This system has the following advantages.
(1) Element having relatively low breakdown voltage compared with the voltage of the direct current source 1 can be used for the respective switching elements since only a fractional voltage of the voltage of the direct current source 1 by the condensers 3 and 4 is applied to the switching elements.
(2) The reactor 9 can be made of smaller capacity since the two switching elements turn-on and turn-off alternately (in general at 180.degree. phase difference) and the frequency of the voltage across the reactor 9 becomes twice with that of the operating frequency of the switching elements.
However, this known system also has the following disadvantages.
(3) Voltage of the condenser 10 i.e. value of the output voltage can not be made higher than the voltage of the direct current source 1.
(4) The load potential varies very suddenly with respect to the negative polarity of the direct current source 1 usually at the ground potential by the voltage of the condenser 3 in accordance with on-off operation of the switching element 6 so that a current flows through a leakage capacitance of the load circuit against the ground potential and this causes unwanted noise.
When a chopper device is used as an auxiliary direct current source of an electric vehicle, the variation range of the input voltage is rather wide in general for instance between DC 900-1800 V so that device may be of a large size by an increase of the required current value to meet a large capacity load in a device having output voltages unable to be higher than the input voltages like the conventional double step-down choppers.
Furthermore, although it is necessary to cut flow of higher harmonics through the capacitance against the ground by decreasing the potential variation against the ground as far as possible in order to avoid troubles by an induced noise against signalling equipments, a problem was involved in the conventional double step-down chopper devices, since a sharp and large potential variation was inavoidable by the on-off operation of the switching elements.